So far the
Free Dining offer has always been for stays in August/September (hurricane/slower months).
The offer usually is announced in April to start booking reservations for Aug/Sept stays, usually first to Disney VISA cardholders, then a couple days later it's offered to the general public.
There are also occasional "Bounceback" offers for guests staying at a WDW resort where they can pre-book stays for the next Aug/Sept Free Dining period and don't have to wait until April, but they have to do that before they check out and put down a deposit while the offer is on - and WDW has only done this 2 times so far - last Nov/Dec '06 for stays this Aug/Sept '07 and this Aug/Sept '07(extended into Oct) for next year Free Dining stays
8/30-9/27/08.
If you want to book Free Dining, just keep watching for people posting here on the DIS - as soon as it shows up, it will show up here! You might have to wait until next April though to see anything for next summer.
Also, when and if it's offered, be sure and check that you are getting the best deal - remember that to get Free Dining, everyone in your party must be on the same package, everyone must purchase at least a one day ticket and you have to pay "rack" rate (no discounts) for the room. There's usually a couple deals that come out at the same time, so make sure that you are actually saving the most $$$ with the Free Dining offer, and that one of the other offers wouldn't save you more in the long run.....